The current mainstream method for producing high purity polysilicon for the electronic and photovoltaic (PV) industries is a combination of metallurgical and chemical. Starting from pure quartz (SiO2), metallurgical grade silicon (MG-Si) is made by carbothermic reduction. This material is then converted into trichlorosilane (TCS, SiHCl3) by reaction with HCl gas. After several purification processes via multiple distillations to remove all metallic and nonmetallic impurities present in MG-Si, the purified TCS gas is used to deposit ultra pure polycrystalline silicon. The processes, collectively called the Siemens Process, are very energy intensive.
The process flow to produce Silicon wafers is the following:MG-Si→Impure TCS→Ultra Pure TCS→PolySi→Crystalline Si→Si Wafer  [1]Single or multi crystal silicon is grown from the polysilicon, leading to monocrystalline silicon rod/cylinder/boule or multicrystalline silicon block. These are then sawed into wafers. Process flow diagrams for the conventional production of MG-Si into Si waters are shown in FIGS. 1 and 2.
Metallurgical grade silicon (MG-Si) is produced by a carbothermic reduction of silica in an arc furnace. This MG-Si material is inexpensive but is of very low (98%-99%) purity. Typical levels of impurities in MG-Si (in parts per million by weight) are: Fe 1550-6500 ppm, Al 1000-4350 ppm, Ca 245-500 ppm, Ti 140-300 ppm, C 100-1000 ppm, O 100-400 ppm, B 40-60 ppm, P 20-50 ppm and traces of such impurities as Mn, Mo, Ni, Cr, Cu, V, Mg and Zr. These residual impurities in MG-Si make it unsuitable for direct use as feedstock for the electronic and PV industries.
TCS is prepared by hydrochlorination of metallurgical grade MG-Si in a Fluidized Bed reactor with a gas stream of hydrogen chloride and hydrogen:Si(s)+3HCl(g)=SiHCl3(g)+H2(g)  [2]The MG-Si is crushed and used in the form 40-200 micron grains. The reaction occurs at 300-350° C. normally without a catalyst. The reaction is highly exothermic (ΔH° reaction=−52 kcal/reaction), and accordingly the feed streams to the fluid bed are controlled. Competing reactions areSi(s)+4HCl(g)=SiCl4(g)+2H2(g)  [3]Si(s)+2HCl(g)=SiH2Cl2(g)  [4]which produce silicon tetrachloride (STC) and Dichlorosilane (DCS). The output of the hydrochlorination reactor is typically 95-97% TCS, 5-7% STC and 1-2% DCS, with all the impurity metal and nonmetal halides.
An alternate way of obtaining TCS from metallurgical grade MG-Si is by reaction with STC and hydrogen in a Fluidized Bed reactor in a process referred to as chlorination according to [5]:Si(s)+3SiCl4(g)+2H2(g)=4SiHCl3(g)  [5]At 500° C. and 35 atm, about 20% TCS is produced with a 1:1 ratio of the reacting gases. The reaction [5] allows the use of pure STC, which is a common and plentiful byproduct of the silicon deposition process, to convert MG-Si to TCS.
The low boiling point of TCS (31.8° C.) and thus its comparatively high volatility lends itself to purification from the impurity halides and especially boron and phosphorus halides, and reduce such and all other impurities down to the parts per billion levels.
After separation of such high boiling metal chlorides as the transition metal chlorides, alkaline earth chlorides, and AlCl3, the trichlorosilane undergoes a multiple purification through fractional distillation, to remove the high boiling volatile components, and then to remove the low boiling volatile components. These distillation purifications of the TCS contribute to more than 50% of the TCS plant operating costs.
Trichlorosilane is the predominant precursor chemical species for industrial polysilicon production due its high silicon deposition rates. High-purity TCS is vaporized, diluted with high-purity hydrogen and introduced into the deposition reactors. The gas is decomposed onto the surface of heated silicon seed rods, electrically heated to about 1100° C., to grow large rods of hyper pure silicon.
The main reactions are:SiHCl3(g)+H2(g)=Si(s)+3HCl(g)  [6]2SiHCl3(g)=SiH2Cl2(g)+SiCl4(g)  [7]SiH2Cl2(g)=Si(s)+2HCl(g)  [8]HCl(g)+SiHCl3(g)=SiCl4(g)+H2(g)  [9]Single pass process conversion efficiency is typical <20%. The stream of reaction by-products which leaves the reactor contains H2, HCl, SiHCl3 and SiCl4. For plant efficiency these are recycled. The SiCl4 is converted to SiHCl3 and along with the unused SiHCl3, the gas stream is recycled and utilized for polysilicon deposition.
More than 95% of the polysilicon for the semiconductor and PV industries is produced through the trichlorosilane route. While the typical polysilicon feedstock produced through the Siemens Process is of ultra high purity necessary and appropriate as demanded for the electronic grade (EG-Si grade, >99.99999%), its purity far exceeds the solar grade (SG-Si) purity requirements (SG-Si grade, >99.999%).
To a limited extent high purity polysilicon is also produced by the thermal decomposition of high purity silane (SiH4) gas in a Siemens reactor. The latter is made from catalytic disproportionation (or redistribution) of high purity trichlorosilane according to:4SiHCl3(g)=SiH4(g)+3SiCl4(g)  [10]Similarly, a lesser purity polysilicon is produced by decomposition of trichlorosilane or silane in a fluid bed reactor.
The polysilicon feedstock is utilized to grow large crystals of silicon for the electronic and PV industries. The electronic industry utilizes mainly the Czochralski process to grow ingots of single crystalline silicon. The photovoltaic industries utilize the Czochralski method only to a small extent. Their main crystal growth process forms multicrystalline silicon by the Heat Exchanger Method (HEM) or Directional Solidification System (DSS). These methods are amenable to grow very large quantities of the silicon crystal in the form of rectangular blocks, and are cost effective for the photovoltaic industry.
The single or multi crystalline silicon ingots or blocks are subsequently sliced into thin wafers by a wire saw or inner diameter saw process. These saw processes also produce significant silicon waste, known as kerf silicon waste, due to the cutting of the silicon ingot or block. While the processed wafer thicknesses are in the range 300 microns, the equivalent kerf loss is 200 microns thick. The semiconductor and photovoltaic industries produce significant quantities of kerf silicon waste during the wafer manufacturing operation. Slicing silicon ingot or block to make wafers is one of the most expensive and wasteful process steps in the silicon value chain, especially in the PV cell manufacturing industry. Kerf loss represents from 40% to 50% of the silicon ingot. This adds significantly to the silicon shortage of the PV industry and in addition, to PV cell manufacturing costs.
While the silicon wafer industry, in collaboration with the wire saw manufacturers, has developed practical processes to recover the cutting fluid and bulk of silicon carbide abrasive from the wafer cutting systems, recovering the silicon powder from the slurry has eluded the PV feedstock industry. The research project, RE-Si-CLE, through the European Community is the only reference for the limited attempt to recycle kerf waste and recover the silicon. This approach parallels the process scheme to directly purify MG-Si by a combination of metallurgical and pyrometallurgical methods. As of today the quality of any such recovered or produced polysilicon falls far short of the minimum specification for solar grade polysilicon.
Silicon is also lost in ingot shaping operations and due to lapping and other chemical mechanical operations on wafers. No serious attempts have been made to recover the silicon powder from these waste streams.
Currently, the primary attempt is metallurgical in nature, that is, to conserve the material aspect of the kerf silicon in the elemental form. However, the purification, separation from SiC and final recovery of Si from the kerf waste is highly dependent on the multitudes of chemical and other process steps. It is very doubtful that such methods will produce high purity PV grade silicon at reasonable cost. It is also certain that such process schemes will contribute to significant process wastes for disposal.
There are other process wastes in the semiconductor and photovoltaic industry that result in silicon powder though not contaminated to the extent that the kerf waste is. Ultra fine silicon powder results as a by-product to the extent of 15-20% of the Fluid Bed process to manufacture high purity electronic or PV grade granular polysilicon. This powder is of high purity, but cannot easily be recycled or used in silicon melting and crystal growth applications because of the finity of the powder. Silicon powder fines are also formed when silicon chunks are crushed to form granule size particles. While the polysilicon industry sometimes practices a simple melting process to recover the silicon dust waste, this process is ineffective, costly and inherently lacks purification of the material. Typical powder metallurgical process schemes to produce compacted silicon shapes and method that uses selective laser wavelengths and energies to densify and melt the silicon powder into granular shapes have been suggested to utilize silicon powder for various applications. As with the kerf silicon, the challenge is recovering the silicon dust waste and produce silicon of the correct or improved purity and process cost. The primary aim with respect to the high purity silicon dust is the conversion to more suitable morphologies for crystal growth at the lowest process cost.